How to make sure only one instance of a Bash script is running at a time?

Question

I want to make a sh script that will only run at most once at any point.

Say, if I exec the script then I go to exec the script again, how do I make it so that if the f

Answer 1

I think you need to use lockfile command. See using lockfiles in shell scripts (BASH) or http://www.davidpashley.com/articles/writing-robust-shell-scripts.html.

The second article uses "hand-made lock file" and shows how to catch script termination & releasing the lock; although using lockfile -l will probably be a good enough alternative for most cases.

Example of usage without timeout:

lockfile script.lock

rm -f script.lock

Will ensure that any second script started during this one will wait indefinitely for the file to be removed before proceeding.

If we know that the script should not run more than X seconds, and the script.lock is still there, that probably means previous instance of the script was killed before it removed script.lock. In that case we can tell lockfile to force re-create the lock after a timeout (X = 10 below):

lockfile -l 10 /tmp/mylockfile

rm -f /tmp/mylockfile

Since lockfile can create multiple lock files, there is a parameter to guide it how long it should wait before retrying to acquire the next file it needs (- and -r ). There is also a parameter -s for wait time when the lock has been removed by force (which kind of complements the timeout used to wait before force-breaking the lock).